A condenser microphone has a microphone unit in which a diaphragm and a backplate are arranged opposedly, and the microphone unit incorporates an impedance converter because the impedance of the microphone unit is very high. As the impedance converter, a field effect transistor (FET) is usually used, but a vacuum tube is used on rare occasions.
A substrate including an audio output circuit of microphone unit is housed in a metallic microphone case. If strong electromagnetic waves are applied from the outside, a high-frequency current caused the electromagnetic waves intrudes into the microphone case and is detected by the impedance converter etc., whereby noise is sometimes generated. The noise caused by the electromagnetic waves is frequently generated when a cellular phone is used in the vicinity of the microphone.
To prevent this noise, for example, the microphone case is connected to a shield coating of a microphone cable via a connecting member. Alternatively, for a line microphone in which a metallic sound tube is housed in a slender and cylindrical microphone case, they are connected electrically, for example, by a leaf spring to form a double shield construction.
However, each of the connecting portions provides a surface contact between metal materials seemingly, but actually provides a point contact microscopically. A contact portion formed by such a point contact exhibits a high impedance in terms of a high-frequency current, and the high-frequency current caused by electromagnetic waves enters into the microphone case from the contact portion. Therefore, it cannot be said that a sufficient shield is formed against electromagnetic waves.
Also, if external stress is applied to the contact portion of this kind, the contact position is displaced frequently, and accordingly the impedance is changed. This suddenly increases the noise caused by electromagnetic waves in some cases. Furthermore, if the displacement of contact position is large, contact noise may occur apart from the noise caused by electromagnetic waves.
Accordingly, a problem of the present invention is that in a condenser microphone of double shield construction having an external cylinder and an internal cylinder, both of which are made of a metal, especially like a line microphone (gun microphone), impedance between shield parts against a high-frequency current caused by electromagnetic waves is always kept low, by which the occurrence of noise caused by electromagnetic waves is prevented.